The MEMS (Micro-Electro-Mechanical System) mirrors and mirror arrays have wide applications in the light process and fiber optic networks such as in optical cross-connect switches, attenuators, wavelength blocker, dynamic gain equalizer, configurable grating and tunable filter etc. The MEMS mirror arrays with high fill factors and one or two axes rotation have particular importance in the wavelength division multiplexing systems. The fill factor is generally defined as the ratio of the active area to the total area in an array. The high fill factor improves the shape of the optical channel and reduces the optical loss in the system. A micromirror with two axes of rotation can provide switching of the optical beam among the channels while avoiding undesirable optical transient cross-talk during switching, and achieving variable optical attenuations.
There are a number of actuation methods for the MEMS micromirror array such as electromagnetic actuation disclosed in U.S. Pat. No. 6,760,145 (Taylor et al.) entitled “Actuator for dual-axis rotation micromirror”, thermal actuation disclosed in U.S. Pat. No. 7,091,057 (Gan et al.) entitled “Method of making a single-crystal-silicon 3D micromirror”, and electrostatic actuation disclosed in U.S. Pat. No. 7,095,546 (Mala et al.) entitled “Micro-electro-mechanical-system two dimensional mirror with articulated suspension structures for high fill factor arrays”. Electrostatic actuation is favored due to its low power consumption and relative simple structure and small footprint.
Existing micromirrors with electrostatic actuation fall into two categories: vertical combdrive type micromirrors and parallel plate type micromirrors. The drawback for conventional vertical combdrive type micromirrors is that is generally fails to form the high fill factor arrays due to its typical gimbaled and framed structure. Since it is difficult to reduce the gap between adjacent micromirrors, it is hard to form a mirror array with high fill factor. An example of this type of MEMS micromirrors is found in U.S. Pat. No. 6,822,776 (Hah et al.) entitled “Scanning micromirror for optical communication systems and method of manufacturing the same”.
It is much easier to form high fill factor mirror arrays based on the parallel plate type of electrostatic actuators. The majority of existing high fill factor micromirror array designs use parallel plate type of electrostatic actuators, such as those taught in U.S. Pat. No. 7,095,546 (Mala et al.) entitled “Micro-electro-mechanical-system two dimensional mirror with articulated suspension structures for high fill factor arrays”, U.S. Pat. No. 6,934,439 (Mala et al.) entitled “Piano MEMS micromirror”, U.S. Pat. No. 6,694,073 (Golub et al.) entitled “Reconfigurable free space wavelength cross connect”, U.S. Pat. No. 6,781,744 (Aksyuk) entitled “Amplification of MEMS motion”, U.S. Pat. No. 6,778,728 (Taylor et al.) entitled “Micro-electro-mechanical mirror devices having a high linear mirror fill factor”, U.S. Pat. No. 7,209,274 (Van Drieenhuizen et al.) entitled “High fill-factor bulk silicon mirrors” and U.S. Pat. No. 7,053,981 (Bleeker) entitled “Lithographic apparatus and device manufacturing method”. The advantage of using a parallel plate electrostatic actuator is that no typical gimbaled structure or frame is required for the design. As such, the gap between the mirrors can be very small to form a high fill factor mirror array.